1. Field of the Invention
This invention relates to digital logic circuits. In particular, the invention relates to oscillator control.
2. Description of Related Art
Oscillators are important elements in digital circuit design because they are the main source of providing clock signals. In Application Specific Integrated Circuit (ASIC) design, the behavior of an on-chip oscillator affects device performance significantly.
An oscillator typically requires a warm-up or start-up time interval when power is first applied in order to reach a stable state. During this stable state the oscillator can provide a stable clock source. If the power is at the same time applied to other circuit elements which rely on the oscillator for performing their operations, an unstable clock signal from the oscillator may lead to incorrect operations. In addition to the power-on reset problem, when an oscillator is temporarily put in a standby or power down mode and subsequently is restored to an operational state, e.g., a wake-up sequence, the restoration process may similarly generate unwanted oscillator""s instability.
One approach to solve this problem includes using a power-on reset circuitry which generates a reset signal long enough to ensure stability has been reached. This technique would still require some way to detect a stable oscillator output. Traditional techniques for detecting a stable oscillator use an analog phase-locked loop (PLL) circuit. However, this technique adds complexity, extra hardware (e.g., extra input/output pins), silicon area, and power consumption.
Therefore, there is a need to have a technique to control an oscillator in an efficient and flexible manner.
The present invention is a method and apparatus to control activation of an oscillator. In one embodiment of the present invention, an oscillator activator includes a trigger circuit, a controller, and a stability generator. The trigger circuit generates a trigger signal based on a reset signal. The controller controls generation of an oscillator signal from a main oscillator based on the reset signal. The stability generator generates a stability indicator signal in response to the trigger signal. The stability indicator signal indicates a stable condition of the main oscillator corresponding to a start-up time of the main oscillator.